Abstract:

A method and computer program product for controlling application of ink
in a printing press include predefining a setpoint color value in a
device-independent color space. An ink metering element is activated with
an ink application value, producing an ink layer thickness, on printing
material, associated with the setpoint color value. An actual color value
from the ink layer thickness on the printing material is determined,
measured or colorimetrically measured in the device-independent color
space. Activation with a changed ink application value based on deviation
of the actual color value from the setpoint color value, produces an ink
layer thickness differing from the produced ink layer thickness. A
necessary ink layer thickness change is calculated based on changes in
the color values in the device-independent color space upon a change in
the ink layer thickness at the point of the actual color value, to
determine the changed ink application value.

Claims:

1. A method for controlling an application of ink in a printing press, the
method comprising the following steps:predefining at least one setpoint
color value in a device-independent color space;activating an ink
metering element in the printing press with an ink application value to
produce, on a printing material, an ink layer thickness associated with
the setpoint color value;determining at least one actual color value of
the ink layer thickness produced on the printing material in the
device-independent color space;carrying out an activation with a changed
ink application value as a function of a deviation of the actual color
value from the setpoint color value, to produce an ink layer thickness
differing from the ink layer thickness produced; anddetermining the
changed ink application value by calculating a necessary ink layer
thickness change as a function of changes in the color values in the
device-independent color space in the event of a change in the ink layer
thickness at a point of the actual color value.

2. The method according to claim 1, wherein the device-independent color
space is the LabI color space.

3. The method according to claim 1, wherein the changes in the color
values in the device-independent color space in the event of a change in
the ink layer thickness are partial derivatives of the color values in
the device-independent color space with respect to the ink layer
thickness.

4. The method according to claim 3, which further comprises determining
the partial derivatives numerically from an assignment of
device-dependent tonal values to a device-dependent color space.

5. The method according to claim 1, which further comprises representing
an assignment of the color values in the device-independent color space
to the changes in the color values in the event of a change in the ink
layer thickness as linking an assignment of the color values in the
device-independent color space to tonal values in a device-dependent
color space with an assignment of the tonal values in the
device-dependent color space to the changes in the color values in the
event of a change in the ink layer thickness.

6. The method according to claim 3, which further comprises calculating
the ink layer thickness change dF in accordance with a
formula:dF=(ΔSi*ΔLabIi)/|ΔS| 2,where
ΔS=(∂L/∂S,
∂a/∂S, ∂b/∂S,
∂I/∂S) at a point
LabIactual,ΔLabI=LabIactual-LabIsetpoint,i counts
off vector components, andsummation is carried out over i=1, 2, 3 and 4.

7. The method according to claim 1, which further comprises carrying out
the production of the ink layer thickness on the printing material in an
offset printing process.

8. The method according to claim 1, which further comprises carrying out
the method for a plurality of colors in a multicolor print.

9. The method according to claim 1, which further comprises carrying out
the method in a plurality of physical zones each being associated with a
respective ink metering element.

10. The method according to claim 1, which further comprises calculating a
plurality of ink layer thickness changes at a plurality of positions on
the printing material and determining a mean value thereof used to
determine the changed ink application value.

11. The method according to claim 1, which further comprises:following a
change in the ink application value, for the activation of the ink
metering element, determining at least one actual color value of the ink
layer thickness produced on the printing material by the ink metering
element activated with the changed ink application value in the
device-independent color space; andcarrying out changes in the ink
application value for the activation of the ink metering element until
the deviation of the actual color value from the setpoint color value
lies within a specific tolerance.

12. A computer program product to be loaded directly into an internal
memory of a digital computer and/or stored on a computer-suitable medium,
the computer program product comprising software code sections for
implementing all of the steps of the method according to claim 1 when the
product runs on the computer.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This application claims the priority, under 35 U.S.C. §119, of
German Patent Application DE 10 2009 013 166.3, filed Mar. 13, 2009; the
prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0002]The invention relates to a method for controlling the application of
ink in a printing press, which includes the steps of predefining at least
one setpoint color value in a device-independent color space, activating
an ink metering element in the printing press with an ink application
value in order to produce an ink layer thickness, on the printing
material, which is associated with the setpoint color value, determining
at least one actual color value from the ink layer thickness produced on
the printing material in the device-independent color space and, in order
to produce an ink layer thickness differing from the ink layer thickness
produced, carrying out activation with a changed ink application value as
a function of a deviation of the actual color value from the setpoint
color value. The invention further relates to a computer program product
for implementing all of the steps of the method according to the
invention.

[0003]During printing, in particular offset printing, fluctuations in the
layer thickness of the printing ink effect changes in the appearance of
the printed printing material, for example the printed paper sheet. In
order to achieve a printed result which corresponds to an intended image,
control or regulation of the application of ink (image regulation) is
commonly performed. A necessary layer thickness change corresponds to a
necessary change in the application of ink, which is typically achieved
through the use of a changed supply of ink. In the simplest case, density
values are measured in a color measuring strip concomitantly printed onto
the printing material, so that the necessary layer thickness changes can
be calculated. Alternatively, individual image points (pixels) within the
printing subject can also be used as measuring points.

[0004]Colorimetric evaluations which are based on spectral measurements
are also widespread. For example, a procedure of that type is described
in U.S. Pat. No. 6,041,708. Color deviations determined in that way are
used to control the application of ink.

[0005]The objective of the image regulation is to control the layer
thickness of the ink in such a way that the current print coincides with
a predefinition, for example a predefined printed example. This
predefinition can also be present in electronic form. Expressed in
another way, the predefinition includes a distribution of
device-independent color values, for example Lab values, which are color
values in a visually uniform equal-interval color space. Density values
as such are insufficient for image regulation inasmuch as their measured
values are not meaningful in the case of overprinted colors. If,
therefore, both the predefinition and the measured values of the current
print are present in the form of device-independent color values, for
example Lab values, then the deviations can be calculated for each image
point.

SUMMARY OF THE INVENTION

[0006]It is accordingly an object of the invention to provide a method for
controlling the application of ink in a printing press and a computer
program product for implementing the method, which overcome the
hereinafore-mentioned disadvantages of the heretofore-known methods and
products of this general type and in which necessary layer thickness
changes are determined for deviations that occur in color values.

[0007]With the foregoing and other objects in view there is provided, in
accordance with the invention, a method for controlling the application
of ink in a printing press, comprising the following steps: At least one
setpoint color value is predefined in a device-independent color space.
An ink metering element in the printing press is activated with an ink
application value to produce an ink layer thickness, on the printing
material, which is associated with the setpoint color value. At least one
actual color value of the ink layer thickness produced is determined on
the printing material in the device-independent color space, in
particular measured, preferably colorimetrically. And in order to produce
an ink layer thickness deviating from the ink layer thickness produced,
activation is carried out with a changed ink application value as a
function of a deviation of the actual color value from the setpoint color
value. In this case, in order to determine the changed ink application
value, a necessary ink layer thickness change is calculated as a function
of the changes in the color values in the device-independent color space
in the event of a change in the ink layer thickness at the point of the
actual color value.

[0008]The changes in the color values in the device-independent color
space in the event of a change in the ink layer thickness are also
designated as sensitivities. In other words, sensitivities indicate how
highly color values change in a device-independent color space when the
layer thickness of the ink changes.

[0009]Advantageously, by using the method according to the invention,
actual value control and, in a further development, actual value
regulation as well, can be implemented. This is significant, in
particular inasmuch as setpoint pre-definitions are frequently derived
from measured data which originate from prints using a color system
deviating from the printing inks used in the printing press, for example
from liquid ink-based or toner-based proofs. In this case, measured data
from specific variables in the device-independent color space, for
example measured data about the infrared value, can be less meaningful,
so that limits are placed on setpoint control.

[0010]In the method according to the invention, a plurality of measured
pixels on the printing material, for example on a printed sheet,
preferably all of the measured pixels, can be taken into account. The
measured pixels can, in particular, also be overprints of a plurality of
printing inks.

[0011]In accordance with another preferred mode of the method of the
invention, the device-independent color space is the LabI color space,
with I standing for the infrared component. In this case, the preferably
colorimetric measurement of the at least one actual color value also
preferably includes a measurement of the infrared component. When the
standard colors cyan (C), magenta (M), yellow (Y) and Black (K) are used,
this component is particularly influenced by the neutral color K.

[0012]In accordance with a further mode of the invention, the changes in
the color values in the device-independent color space in the event of a
change in the ink layer thickness can be represented by the partial
derivatives of the color values in the device-independent color space
with respect to the ink layer thickness. In particular, the partial
derivatives can be determined numerically from an assignment of the
device-dependent tonal values to the device-dependent color space.

[0013]In accordance with an added mode and practical implementation of the
method of the invention, the assignment of the color values in the
device-independent color space to the changes in the color values in the
event of a change in the ink layer thickness can be represented as
linking an assignment of the color values in the device-independent color
space to tonal values in a device-dependent color space with an
assignment of the tonal values in the device-dependent color space to the
changes in the color values in the event of a change in the ink layer
thickness.

[0014]In accordance with an additional preferred mode of the method of the
invention, the ink layer thickness change dF is calculated in accordance
with the formula:

dF=(ΔSi*ΔLabIi)/|ΔS| 2,

[0015]where ΔS=(∂L/∂S,
∂a/∂S, ∂b/∂S,
∂I/∂S) at the point LabIactual,
ΔLabI=LabIactual-LabIsetpoint, i counts off the vector
components and summation is carried out over i=1, 2, 3 and 4.

[0016]As an alternative to the error ΔLabI, it is also possible to
calculate with the difference ΔLab without any infrared component,
that is to say also to set ΔI=0, for example. This can be the case,
for example, if the setpoints are present only as Lab values. Regulation
modified in this way can also be designated as proof regulation.

[0017]In accordance with yet another mode of the invention, which is
particularly significant in practice, the method is used in an offset
printing press. Stated in another way, the production of the ink layer
thickness on the printing material takes place in an offset printing
process.

[0018]In accordance with yet a further mode of the invention, which
represents a first further development of the method, the method is
carried out for a plurality of colors in a multicolor print. A second,
additional or alternative further development resides in carrying out the
method according to the invention in a plurality of physical zones, to
which an ink metering element is assigned in each case. Stated in another
way, this can involve a printing press having a zonal inking unit.

[0019]In accordance with yet an added or alternative mode of the method of
the invention, a plurality of ink layer thickness changes can be
calculated at a plurality of positions on the printing material and a
mean value thereof can be determined, which is used to determine the
changed ink application value.

[0020]In accordance with yet an additional mode of the method of the
invention, which can also be developed further to form a regulating
method: Following a change in the ink application value, for the
activation of the ink metering element, at least one actual color value
of the ink layer thickness produced on the printing material by the ink
metering element activated with the changed ink application value is
determined in the device-independent color space. Changes in the ink
application value for the activation of the ink metering element are
carried out until the deviation of the actual color value from the
setpoint color value lies within a specific tolerance.

[0021]The calculations can be carried out in the preliminary part of a use
of the method according to the invention and then stored in an ICC
profile. Then, using a speed-optimized Color Management Module for each
actual value of an image pixel, a control variable can advantageously be
calculated for each color so that, as opposed to color measuring strip
regulation, the printed result can be achieved in an overall optimal
manner.

[0022]With the objects of the invention in view, there is concomitantly
provided a computer program product associated with the concept of the
invention. This computer program product can be loaded directly into the
internal memory of a digital computer and/or stored on a
computer-suitable or readable medium. According to the invention, the
computer program product includes software code sections with which all
of the steps of a method according to the invention can be implemented
when the product runs on a computer.

[0023]The digital computer can, in particular, be a control computer of a
printing press or a computer of a colorimetric measuring system for
printed products from a printing press.

[0024]The method according to the invention can be used, in particular,
for sheet-fed printing presses. The printing press can operate in
accordance with a direct or indirect planographic printing process, in
particular an offset printing process.

[0025]Other features which are considered as characteristic for the
invention are set forth in the appended claims.

[0026]Although the invention is described herein as embodied in a method
for controlling the application of ink in a printing press and a computer
program product for implementing the method, it is nevertheless not
intended to be limited to the details provided, since various
modifications and structural changes may be made therein without
departing from the spirit of the invention and within the scope and range
of equivalents of the claims.

[0027]The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be best
understood from the following description of specific embodiments when
read in connection with the accompanying example.

DETAILED DESCRIPTION OF THE INVENTION

[0028]Referring now in detail to the invention, further advantages and
advantageous embodiments and developments thereof will be described below
with reference to an example of calculating an ink layer thickness
change.

[0029]A transformation TR1 from the device-dependent color space CMYK to
the device-independent color space LabI is calculated with the aid of a
color model, for example the color model used in the CPC24 module from
Heidelberger Druckmaschinen AG. To this end, the CMYK space is sampled at
equidistant reference points. The LabI values calculated by using the
color model are stored in a table. For example, reference points are
generated at 20% steps. The resultant table then has a size of 6 4*4
values. If interpolation is carried out four-dimensionally, a LabI value
in the range from 0% to 100% can be calculated for each CMYK value with
the aid of this table.

[0030]An inverse transformation TR2 from the device-independent color
space LabI to the device-dependent color space CMYK is then calculated.
For the purpose of storage in an ICC profile, the appropriate CMYK values
are also calculated in this case at equidistant reference points in the
LabI space. Alternatively, in an ICC profile it is also possible to use
four one-dimensional input curves and output curves, so that it is also
possible to sample non-equidistantly. There are various mathematical
methods for the actual calculation of the inverse transformation TR2. For
instance, LabI values can be looked for by variation of the CMYK values
and interpolation of the values in the transformation TR1. As an
alternative to this, local 4×4 matrices with appropriate weighting
can be inverted. Outside of the space predefined by the LabI values of
the transformation TR1, suitable interpolation must be carried out. The
result is an ICC profile ICC1, which transforms from the LabI space into
the CMYK space.

[0031]There follows a calculation of the transformation of the CMYK values
to sensitivities ΔS. The 4×4 matrices ΔS are then
calculated at equidistant reference points in the four-dimensional CMYK
space. For this purpose, through the use of the color model, a LabI value
(LabI_c0) is calculated from the current CMYK value. The ink layer
thickness of the color C is then increased by a specific value, for
example by 1%, and a LabI value (LabI_c1) is calculated. The procedure is
carried out in a corresponding way for the other colors M, Y and K.

[0032]The differences (LabI_c1-LabI_c0), (LabI_m1-LabI_m0),
(LabI_y1-LabI_y0), (LabI_k1-LabI_k0) are stored as a ΔS matrix. In
other words, in ΔS there are the numerical partial derivatives
dLabI/dS for the four colors C, M, Y and K, that is to say
∂L/∂S_c, ∂a/∂S_c,
∂b/∂S_c, ∂I/∂S_c,
∂L/∂S_m, ∂a/∂S_m, . .
. In total, there are 16 values for each reference point.

[0033]In this case, it is significant that the sensitivities depend on the
combination of the printing ink proportions, for example quantified as
screen percentage values. The result is therefore different sensitivities
for the case when a printing ink is printed on its own, for example with
40% area coverage, than for the case when at least one other color, for
example two other colors, have previously also been printed at the point.
Stated in another way, the sensitivities at each point in the color space
(for example a four-dimensional space in the case of four printing inks)
are as a rule different.

[0034]The result is an ICC profile ICC2, which assigns the sensitivities
AS to CMYK values.

[0035]The ICC profiles ICC1 and ICC2 are calculated together using a Color
Management Module, for example using that marketed by Heidelberger
Druckmaschinen AG. In this case, the number of reference points can also
be different, since interpolation between the reference points is carried
out. The result is an ICC profile ICC_Combi, which assigns the
sensitivities ΔS to LabI values.

[0036]In order to calculate a necessary layer thickness change in an ink
which ensures that the ink layer thickness is changed in such a way that
the desired setpoint in the LabI space is achieved, a calculation is
carried out as follows for each pixel: For each actual value in the LabI
space, through the use of the ICC profile ICC_Combi, the sensitivities
ΔS are determined using a Color Management Module, for example
using that marketed by Heidelberger Druckmaschinen AG. The color error
vector ΔLabI of the actual LabI value and of the LabI setpoint is
calculated. The ink layer thickness change dF for an ink for one pixel is
then given by:

dF=(ΔSi*ΔLabIi)/|ΔS| 2,

[0037]where ΔS=(∂L/∂S,
∂a/∂S, ∂b/∂S,
∂I/∂S) at the point LabIactual,
ΔLabI=LabIactual-LabIsetpoint, i counts off the vector
components and summation is carried out over i=1, 2, 3 and 4. Stated in
another way, the ink layer thickness change for a specific ink is the
scalar product of the unit vector in the direction of the vector ΔS
for the specific ink with the color error vector ΔLabI divided by
the magnitude of the vector ΔS for the specific ink.

[0038]In a preferred embodiment, a mean ink layer thickness change is
calculated by an average being calculated over a plurality of pixels or
all of the pixels of the zone, for example the arithmetic mean is
calculated. In this case, it is also possible to take into account
diverse other weightings which increase the precision of the calculation
of the necessary ink layer thickness change.

[0039]As a result of the use of ICC profiles and a Color Management
Module, the sensitivities at the point of the actual values can
advantageously be calculated at high speed for each pixel. The control
variables for each printing ink can be determined therewith. Actual value
image regulation can be implemented.